Electrochemical capacitors (known as ultracapacitors or supercapacitors) based on electrical double layer (EDL) charge accumulation hold promise for a wide range of applications, including portable electronics, uninterruptable power sources, medical devices, load leveling, and hybrid electric vehicles. Conventional organic electrolytes used in EDL supercapacitors contain a mixture of a solvent and a salt. However, the exclusive use of organic electrolytes may limit the broadening of the supercapacitors' commercial application base, since solvents such as acetonitrile have issues associated with their flammability at elevated temperatures, as well as their toxicity and environmental impact. Alternative electrolytes based on solvent-free ionic liquids possess several advantages over organic ones, including higher operating voltage windows (>3V vs. ˜2V), lower toxicity, negligible vapor pressure, and much better thermal stability. Unfortunately, supercapacitors based on ionic liquids normally perform well only at temperatures near or above 60° C. The room temperature performance, which is an essential prerequisite for most commercial applications, remains poor due to ionic liquid's high viscosity and low ionic diffusivity. Moreover, large cation and anion sizes limit the usefulness of conventional microporous activated carbon electrodes since the ions either literally do not fit into pores or become diffusion limited at required scan rates. It is only with custom tailored eutectic ionic liquids that lower temperature performance may be achieved using carbon nanotubes and carbon onions.
Activated carbons, templated carbons, carbon nanofibers, carbon nanotubes, carbide-derived carbons, and graphene have been intensively investigated for supercapacitor electrode applications. Among them, activated carbons have been successfully developed as electrodes for commercial supercapacitor devices. Commercial high surface area “electrode grade” activated carbons usually possess moderate gravimetric capacitances in the range of 100-120 F g−1 in an organic electrolyte. Depending on the commercial source, activated carbons are derived from pyrolysis of agricultural wastes or from the coking operation during petroleum refining. Recently, outstanding specific capacitances of 200-300 F g−1 in organic electrolyte or ionic liquid have been reported by employing improved activated carbon electrodes, with tailored pore size distributions. However the power characteristics of many of these carbons remain limited due to an intrinsically high fraction of microporosity, which in turn limits pore accessibility of the electrolyte ions at high scan rates.
It is becoming well understood that the key to achieving high power in porous electrodes is to reduce the ion transport time. The ion transport time (τ) can be expressed by the equation of τ=12/d, where 1 is the ion transport length and d is the ion transport coefficient. From that vantage, carbons with open 2D type morphology possess an intrinsic advantage over particulate type systems since the ion transport length is significantly shortened in the thin dimension. Therefore nanomaterials based on graphene and their hybrids have emerged as a new class of promising high-rate electrode candidates. Activated graphene, curved graphene, laser-scribed graphene, ultrathin planar graphene and sponge-like graphene, which possess large open and relatively flat adsorption surfaces in addition to high in-plane electrical conductivity, have excellent electrochemical performance with energy-power combinations often much superior to activated carbons. Widely used methods for synthesis of graphene-like materials include modified Hummers method, chemical vapor deposition, and microwave synthesis. Unfortunately, even the most economically produced graphene-like material is nowhere near cost competitive with petroleum or biowaste derived carbons achieved via simple pyrolysis or hydrothermal methods. Biomass, which mainly contains cellulose, hemicelluloses, and lignin biopolymers, is widely utilized as a feedstock for activated carbon production.
Hemp (Cannabis sativa L.) has been cultivated for centuries since it grows quickly without any special requirements for climate, pesticides, or fertilizer. Besides the ancient applications for rod, sails, and clothing, hemp is currently being used for paper, building materials, food, medicine, oil, fuel, and in the plastics industry. Conventionally, carbonized hemp fiber has also been recently prepared, with activation being achieved via water, ZnCl2, and H3PO4. Though the products were not fully tested for electrochemical energy storage it is expected that they would perform entirely analogously to other forms of pyrolyzed carbon particulates.